Syn gas generated from coal gasification processes or the like contains large amounts of CO, moderate amounts of hydrogen (H2), plus oxidants, such as water vapor (H2O) and carbon dioxide (CO2). The oxidant levels may vary, depending upon the processes used to generate the syn gas. For example, if a melter-gasifier is used to generate the syn gas, whereby molten iron is produced as a product or by-product, an intermediate product of the pre-reduced iron is made using the melter gases and fed back to the melter, and off gases from the pre-reduction unit are exported for further use, the CO2 content may be quite high (>25%). This syn gas may have a CO content of >40% and a H2 content of about 15%. In order to use this syn gas for direct reduction (DR), the H2/CO ratio should be near 1.0 and the CO2 less than about 5%.
Coal gas or syn gas with about 54% CO, 30% H2, and 11% CO2 has been used. This syn gas has been added to a recirculated top gas from the DR furnace, with subsequent CO2 removal, humidification, heating to near reduction temperature, and reacting or shifting (CO+H2O<=>CO2+H2) in a reactor immediately upstream of the DR furnace. A reducing gas with about 43% H2 and 41% CO, for a H2/CO ratio of about 1.05 results, which is suitable for the DR of iron. Disadvantageously, all of the reducing gas has to flow through the shift reactor, requiring a relatively large shift reactor. The high shift reactor temperatures required (about 800 degrees C.) also significantly increase equipment cost.
Other methods and systems shift a high-CO content export gas directly from a melter-gasifier, before using the gas in a DR plant circuit. The export gas is shifted in one or two reactors before CO2 is removed. Recirculated top gas from the DR furnace is added to the shifted export gas prior to CO2 removal, or it may be added after CO2 removal. With these methods and systems, it is desirable to have a gas composition after CO2 removal with a H2/CO ratio of between about 2/1 and 20/1. The first stage shift reactor operates at about 490 degrees C., and the second stage shift reactor operates at between about 360 and 390 degrees C.
Further methods and systems teach similar processes, with the addition that CO2-laden tail gas from the CO2 removal unit is used as fuel for generating the steam required by the shift reactor. The shift reactor is used directly on the export gas from the melter-gasifier.